This project concerns a comprehensive evaluation of lymphoid cell subpopulations in children with acute lymphocytic leukemia (ALL). Major and minor subpopulations of blasts and lymphocytes from untreated patients will be identified and quantitated by determination of lymphocyte surface receptors. Functional activity of lymphocytes and blast cells will be evaluated by in vitro proliferation in response to mitogens, antigens and allogeneic cells as well as by nonspecific cytotoxicity against normal and malignant human cells. The capacity of leukemic cells to regulate lymphocyte function will be assessed by capacity to alter in vitro proliferation of autologous remission and normal allogeneic lymphocytes. These studies will be carried out using peripheral blood and bone marrow lymphoid cells as well as subpopulations separated as a function of cell surface receptors or velocity sedimentation. Longitudinal evaluation of lymphoid cell population will be carried out on selected patients throughout the course of treatment. The information obtained will be integrated with clinical evaluation of the patients under study. It is anticipated that this approach will provide a rational basis for determining clinically relevant subcategories in ALL as well as insight into the pathogenesis of ALL.